Imports System
Module CommonLettersProgram
'
' Efficient algorithm using bitmasking:
' ------------------------------------
' Each letter 'a'..'z' is represented by one bit in a 32-bit Integer.
'
' bit 0 -> 'a'
' bit 1 -> 'b'
' ...
' bit 25 -> 'z'
'
' For each word:
' - Set the bit corresponding to each letter.
' Then:
' - Intersect all bitmasks using bitwise AND.
' The remaining bits represent letters common to all words.
'
' This avoids sorting, dynamic memory, and repeated scanning.
'
' Convert a word into a bitmask of its letters
Function WordToMask(word As String) As Integer
Dim mask As Integer = 0
For Each c As Char In word
If c >= "a"c AndAlso c <= "z"c Then
mask = mask Or (1 << (Convert.ToInt32(c) - Convert.ToInt32("a"c))) ' set bit for this letter
End If
Next
Return mask
End Function
' Compute common letters across all words
Function CommonLetters(words As String()) As Integer
If words.Length = 0 Then Return 0
Dim common As Integer = WordToMask(words(0))
For i As Integer = 1 To words.Length - 1
common = common And WordToMask(words(i)) ' bitwise intersection
Next
Return common
End Function
' Print letters represented by a bitmask
Sub PrintMaskLetters(mask As Integer)
For i As Integer = 0 To 25
If (mask And (1 << i)) <> 0 Then
Console.Write(Convert.ToChar(Convert.ToInt32("a"c) + i) & " ")
End If
Next
Console.WriteLine()
End Sub
Sub Main()
Dim words As String() = {
"algebraic",
"alphabetic",
"ambiance",
"abacus",
"metabolic",
"parabolic",
"playback",
"drawback",
"fabricate",
"flashback",
"syllabic"
}
Dim resultMask As Integer = CommonLetters(words)
Console.WriteLine("Common letters across all words:")
PrintMaskLetters(resultMask)
End Sub
End Module
'
' run:
'
' Common letters across all words:
' a b c
'
